Definatly sounds like a common case of black ich. Black ich is not as large as white ich, and much much less distructive. It will look like someone dusted the fish in really really fine black powder. The best way to treat is with a quarintine tank, and formalin. After the first bath ich may appear to be gone, but I reccommend keeping them in the QT for the full 4-6 treatments to allow the ich to die off in your tank (fast lifecycle than white ich), and to be sure it's completely off your fish so you wont have to do this again...anytime soon.